Telephone switching systems typically constitute a switching network for temporarily establishing transmission paths between various pairs of subscribers. When one subscriber calls another, a path is established between the two subscribers which is an actual electrical path or which, in more sophisticated systems, acts like one. Traditionally, the switching network has been designed to switch analog voice signals between the subscribers using known pulse code modulated (PCM) time division multiplexing (TDM) techniques.
In the switched public network telephone system, a two wire subscriber loop, often called a "twisted pair", connects a subscriber's telephone to a central office switch The subscriber loop carries an analog voice signal. The central office switch pulse code modulates the voice signals from active loops and time multiplexes these pulse code modulated signals to effect switching between active subscriber loops and establish a point-to-point connection between two subscribers In addition, TDM trunk lines interconnect the central offices with each other to effect switching between subscribers served by different central offices.
Increasing levels of interoffice traffic have been accompanied by increasing use of digitized interoffice trunks. The digitization of interoffice communications has had the effect of allowing more TDM channels to be carried on each TDM trunk so that each central office may serve increasing numbers of subscribers and so that higher quality service may be provided to the subscribers. Of course, the central office PCM-TDM switches have to interface with the digital interoffice network. Accordingly, there has been a trend to digitize the central office switches. However, this trend has not been universal. Therefore, many central offices still retain the PCM-TDM switches.
Irrespective of the character of the central office switch, the subscriber loop has, in nearly all public telephone systems, remained a strictly analog portion of the existing switched public network. Therefore, although digitization of switching and communications at the central office has allowed the quality of the basic service to continually improve, the subscriber loop portion of the typical telephone service has remained relatively unchanged.
Recently, there have been many proposals and attempts to digitize the subscriber loop. A primary impetus behind digitizing the subscriber loop is to provide the subscriber with integrated services in a fully digital telephone network. Such integrated services would include, for example, the ability to have a mix of medium speed and low speed data communication channels which could be in use simultaneously with one or more voice communication channels. The proposed integrated services digital networks (ISDN's) would operate on existing two wire, twisted pair subscriber loops, although other vehicles such as optical fiber are also possible. A most apparent advantage of ISDN is that the existing network of subscriber loops would not have to be replaced, nor would they have to be supplemented by additional twisted pair loops to provide the integrated services. Another advantage of the ISDN approach is that the subscriber loop would not have to be selected for either voice or data communications to the exclusion of the other. In the analog loop, data communications must be coupled through a modem to the exclusion of voice communications.
As proposed in typical ISDN systems, the low speed data communication channels would be useful for vendors of certain services, such as home security and alarm services. The medium speed data communication channels would be useful for connection to packet assembler/disassembler (PAD) vendors, such as the Telenet and Tymnet services. In most existing telephone systems, if such services are provided, all of these vendors must interface through the existing subscriber loop to the exclusion of the voice channel or through an additional subscriber loop in which a point-to-point telephone connection can be established. Integrated services digital networks, on the other hand, provide the subscriber far greater flexibility than is now possible with the existing analog subscriber loop. An example of a telephone system providing integrated voice and data services over twisted pair subscriber loops is described in copending application entitled Telephone System, Ser. No. 711,083, filed Mar. 12, 1985, which is assigned to the assignee of the present invention and is incorporated herein by reference.
In designing an integrated services digital network, it becomes necessary to provide at the central office the capability of demultiplexing the digitized voice and data channel information received from each subscriber loop and to switch each voice and data channel to the intended recipient. Since many central office switches cannot switch pure digital data, it often becomes necessary first to terminate the subscriber loop to perform a first order demultiplexing which switches out and converts the voice bits received from the subscriber loop to pulse code modulated (PCM) analog bits for application to the existing central office switch. The PCM encoded voice may then be switched and routed in a conventional manner. The termination may also take the data bits from the subscriber loops and multiplex these onto a TDM bus. The TDM data containing several data channels from each subscriber loop must then be switched to other TDM buses which serve the data services vendors or other intended recipients. To avoid duplication of hardware, it is desirable that the data be switched in a full duplex mode so as to be capable of switching concurrent bidirectional data between the subscriber loops and data services vendors or other intended recipients.